The substitution of transparent components of mineral origin with transparent components made of moulded plastic material provides a functional alternative at low production cost, and complete independence from certain supply sources of up-market transparent materials.
However, moulded transparent parts are, on the one hand, relatively thick since it is difficult to mould very thin parts with adequate reproducible quality. On the other hand, the wear resistance, and particularly scratch resistance of these known plastic materials is very mediocre. The use thereof for the external components of timepieces—the middle part of the case, bezel or crystal—is thus limited in time.
This low wear resistance also makes it impossible to produce components with sharp angles, which would become blunt too quickly over time on contact with the user, his clothes, and ordinary everyday objects.
It is useful, in horology, to have components having a high surface hardness, in particular for those external timepiece components, which are subject to stresses from the environment and the user. These components must also have certain physical properties of elasticity, rigidity or conversely flexibility, or particular optical properties, to enhance certain displays or components, or conversely to conceal certain parts of the timepiece.
EP Patent No 1666225 in the name of BAYER MATERIAL SCIENCE CLC describes a method for fabricating a plastic component which is moulded in a first chamber, and coated in a second cavity of the mould, in which the coating material is compressed.
JP Patent Application No 58080587A in the name of SUWA SEIKOSHA describes an improvement to watch cases, in terms of resistance and dimensional precision, by the use of fibre glass treated with acrylic silane and mixed with a synthetic UV hardening resin, such as spirane resin, with a photo-polymerisation initiator agent such as benzophenone. These watch cases are moulded in elastic synthetic rubber or silicon moulds, which are fixed to glass plates and then UV-irradiated for 30 seconds to 30 minutes depending on the intensity of radiation.